1. Field of the Invention
This invention relates to transition metal catalysts having at least one heterocyclic ligand, and in particular, to transition metal catalysts useful for olefin coupling and polymerization.
2. Background Art
The chemical industry uses a wide variety of transition metal complexes as catalysts for organic reactions. Olefin polymerization is an important example of such a reaction. While conventional Ziegler-Natta catalysts continue to dominate the industry, highly active metallocene and non-metallocene single-site catalysts that give new polymers with narrow molecular weight distributions, low densities, and good comonomer incorporation are emerging.
Transition metal complexes used to polymerize olefins are normally non-zero-valent metals (e.g., Ti4+, ZR4+, Sc3+) surrounded by anionic ligands (e.g., chloride, alkyl, cyclopentadienyl) that satisfy the valency of the metal and often improve the solubility of the catalyst in the reaction medium. Anionic ligands can dramatically affect catalyst activity and polymer properties. Thus, a catalyst structure can be fine-tuned to give polymers with desirable properties. Furthermore, the anionic ligand will affect the stability of the transition metal complexes.
Metallocene polymerization catalysts contain one or two cyclopentadienyl groups as anionic ligands. These serve to stabilize the active catalytic species, modulate the electronic and steric environment around the active metal center, and maintain the single-sited nature of the catalyst. Polymers with narrow molecular weight and composition distributions are formed from these metallocene catalysts. Such complexes frequently contain cyclopentadienyl groups. Putting substituents on the cyclopentadienyl ring, for example, changes the geometry and electronic character of the active site.
Another class of anionic ligands is those which are heteroatomic ring ligands which are isolobal to the cyclopentadienyl ring; that is, the orbital interaction of the metal with the ligand is similar in both cases. Examples of such ligands are boraaryl (see, e.g., U.S. Pat. No. 5,554,775), pyrroyl and indolyl anions (U.S. Pat. No. 5,539,124), azaborolinyl groups (U.S. Pat. No. 5,902,866), indole-indenyl groups, phospholyl anions, and tris(pyrazolyl)borate anions.
Frequently, transition metal catalyst exhibit sensitivity to moisture and oxygen and poor stability during storage. This poor sensitivity and stability is often caused by a halogen attached to the transition metal or by an alkyl or aryl ligand that is attached to the transition metal of the catalyst by a bond to a carbon atom.